1. Field of the Invention
The present invention pertains to interval velocity analysis and more particularly to interval velocity analysis and depth migration which uses common reflection point (or common depth point) gathers.
2. Related Prior Art
Migration techniques operate by broadcasting all recording events on a pre-stack input trace to all possible subsurface locations from which the reflection event could have originated. FIG. 1 shows the result of migrating single trace.
When more traces are broadcasted, images begin to appear at places where broadcasted events are reinforcing each other. At places where no images are expected, the corresponding broadcasted events should cancel each other. FIG. 2 shows the result of migrating twelve shots, each consists of eighty traces. The images are forming on the left edge of the figure, and cancellation is apparent shallow in the section.
FIG. 3 shows the result of migrating a complete line of 33, 000 traces. Over most of the migrated section we have clean images, which means the broadcasting methods works well. But between 16000 to 19000 feet there are artifacts, or migration arcs, which indicates incomplete cancellation of unwanted events. This occurs primarily in regions where the desired image is weak. A strong event recorded on either side or below a weak event can arc into this region and interfere with the weak image.
Prior art has disclosed many methods for processing seismic data which are used with common reflection point (CRP) gathers. Normal moveout correction is primarily used to compensate for noise or undesirable effects. As indicated previously, a significant problem with processing and migrating seismic data in the application of CRP analysis occurs when strong events occur near weak events. Examples of processing methods which include migration and normal moveout correction are as follows.
United States Registration number H482 "Seismic Migration Method" (John R. Berryhill et al.) relates to a seismic data processing method in which seismic traces are subjected to Fourier transformations. The coefficients of the Fourier-Transformed traces are subjected to a recursive FK migration operation. The migrated traces are thereafter inverse-Fourier-transformed. Each trace contains a signal resulting from reflection of a seismic signal at a location within the earth, and each trace is associated with at least one point in a two-dimensional spatial grid (x,y). When displayed, the processed seismic data represents the position within the earth of whatever caused the reflection. The method may be employed to process stacked seismic traces, each associated with a single point (x,y) in the grid, or may be employed to process unstacked seismic traces, each associated with both a seismic source location (X.sub.s,Y.sub.s) and a different seismic receiver location (X.sub.r,Y.sub.r) in the grid. In performing the method, the earth is modeled as a stack of M horizontal layers, each characterized by a seismic wave velocity. The recursive FK migration step is iterated M-1 times for each trace, where part of the output of each iteration is stored and part discarded.
U.S. Pat. No. 4,802,147 titled "Method for Segregating and Stacking Vertical Seismic Profile Data in Common Reflection Point Bins" (George P. Moeckel) relates to a method for segregating and stacking vertical seismic profile data. The offset difference between the well location and the position of the source is divided into equal segments. Vertical seismic profile moveout corrected data is placed in common reflection point bins and stacked.
U.S. Pat. No. 4,813,027 titled "Method and Apparatus for Enhancing Seismic Data" (Hans Tieman) relates to a method and apparatus for stacking a plurality of seismic midpoint gathers to provide a pictorial representation of seismic events. The approximate propagation velocity, corresponding to a selected event in a common midpoint gather, is determined by summing the common midpoint gather using first and second weights to provide respective first and second weighted sums over an offset based on an estimated velocity corresponding to the event. A velocity error value indicative of the approximate error between the estimated velocity and the actual velocity is developed from the sums. The common midpoint gather is then restacked in accordance with the determined propagation velocity to provide an enhanced pictorial representation of the seismic event. The first and second weighted sums are taken over a time window centered upon an estimated zero offset travel time for the event. The first and second weights can be selected to provide rapid, slow or intermediate convergence upon the true velocity. The velocity error value is determined as a function of the deviation of the peak of the first weighted sum from the center of the time window, relative to the deviation of the peak of the second weighted sum from the center of the time window. Alternatively, the velocity error value is determined as a function of the deviation of the peak of the cross-correlation of the first and second weighted sums from the center of the time window.
U.S. Pat. No. 4,241,429 titled "Velocity Determination and Stacking Process from Seismic Exploration of Three Dimensional Reflection Geometry" (Marvin G. Bloomcuist et al.) relates to a method for determining the dip and strike of subsurface interfaces and average propagation velocity of seismic waves. In seismic exploration, linear, multiple fold, common depth point sets of seismograms with three dimensional reflection geometry are used to determine the dip and strike of the subsurface reflecting interfaces and the average velocity of the path of the seismic energy to the reflecting interface. The reflections in each set appear with time differences on a hyperbola with trace spacings determined by the source receiver coordinate distance along the lines of exploration. The offset of the apex of this hyperbola is determined from a normal moveout velocity search of the type performed on two dimensional common depth point (CDP) sets. This search identifies the correct stacking velocity and hyperbola offset which are used to determine dip, strike and average velocity.
U.S. Pat. No. 4,766,574 titled "Method for Depth Imaging Multicomponent Seismic Data" (Norman D. Whitmore, Jr., et al ) relates generally to a method of geophysical exploration. This method may be used for imaging multicomponent seismic data to obtain depth images of the earth's subsurface geological structure as well as estimates of compressional and shear wave interval velocities. In particular, measures are obtained of imparted seismic wavefields incident on reflecting interfaces the earth's subsurface and of resulting seismic wavefields scattered therefrom. The incident and scattered seismic wavefields are employed to produce time-dependent reflectivity functions which are representative of the reflecting interfaces. By migrating the time-dependent reflectivity functions, better depth images of the reflecting interfaces can be obtained. For a dyadic set of multicomponent seismic data, the dyadic set is partitioned in order to separate the variously coupled incident and reflected wavefields in the recorded multicomponent seismic data. The incident and reflected wavefields are cross-correlated to form reflectivity functions that are time-dependent. These time-dependent reflectivity functions are then iteratively migrated according to a model of wavefield velocities of propagation to obtain estimates of the compressional and shear wave interval velocity. The migrated reflectivity functions can then be stacked to produce depth images of the earth's subsurface geological structures.
U.S. Pat. No. 4,802,146 titled "Method for Moveout Correction and Stacking Velocity Estimation of Offset VSP Data" (George P. Moeckel) relates to a moveout correction process and stacking velocity estimation process to permit stacking of vertical seismic profile (VSP) data. The primary reflection time is determined by using the two-way travel time, the root mean square velocity of acoustic pulses in the formation and the first arrival time of direct path acoustic pulses.
U.S. Pat. No. 4,736,347 titled "Multiple Stacking and Spatial Mapping of Seismic Data" (Bernard Goldberg et al.) relates to a method for determining the dip of subsurface formations and the apparent acoustic velocity. Seismic traces are stacked in a plurality of orthogonal measures to form multiple stacked traces at a positive offset. The stacking process determines the apparent velocities as functions of the travel time at the positive offset. The interval acoustic velocity of the first layer is then determined from knowledge of surface topography, source-receiver offset, two-way travel times and the first reflector apparent velocities. The first layer velocity information enables the incident and emergent angles of the raypaths at the surface to be calculated, as well as enabling the dip angles and spatial coordinates of the reflection points on the first reflecting boundary to be determined. Seismic data corresponding to the second reflecting boundary are then mapped spatially to the first reflecting boundary by ray tracing and by calculating the apparent velocities at the first boundary. The process is repeated for each succeedingly deeper boundary. The derived acoustic velocity model of the earth is displayed as a stacked seismic section in spatial coordinates. This process may be applied to obtain earth models and seismic sections in both two and three dimensions.
U.S. Pat. No. 4,745,585 titled "Method of Migrating Seismic Data" (Kenneth L. Larner) relates to a method for migrating seismic data in steps where each step is a portion of the total migration. Seismic data is passed through a preselected number of migration stages. During each stage, data is migrated a plurality of times, where the migration-velocity function is a minor fraction of the velocity required to fully migrate the data in a single stage. The cascaded migration is used to migrate data having steeply-dipping events.
Although prior art has shown ways to correct for specific migration problems, the problem of strong events cancelling or overriding weak events still remains.